The hip flexor muscles are major contributors to lumbar spine stability. Tight hip flexors can lead to pain in the lumbar spine, and hence to an impairment in performance. Moreover, sedentary behavior is a common problem and a major contributor to restricted hip extension flexibility. Stretching can be a tool to reduce muscle tightness and to overcome the aforementioned problems. Therefore, the purpose of this systematic review with meta-analysis was to determine the effects of a single hip flexor stretching exercise on performance parameters. The online search was performed in the following three databases: PubMed, Scopus, and Web of Science. Eight studies were included in this review with a total of 165 subjects (male: 111; female 54). In contrast to other muscle groups (e.g., plantar flexors), where 120 s of stretching likely decreases force production, it seems that isolated hip flexor stretching of up to 120 s has no effect or even a positive impact on performance-related parameters. A comparison of the effects on performance between the three defined stretch durations (30-90 s; 120 s; 270-480 s) revealed a significantly different change in performance (p = 0.02) between the studies with the lowest hip flexor stretch duration (30-90 s; weighted mean performance change: -0.12%; CI (95%): -0.49 to 0.41) and the studies with the highest hip flexor stretch duration (270-480 s; performance change: -3.59%; CI (95%): -5.92 to -2.04). Meta-analysis revealed a significant (but trivial) impairment in the highest hip flexor stretch duration of 270-480 s (SMD effect size = -0.19; CI (95%) -0.379 to 0.000; Z = -1.959; p = 0.05; I2 = 0.62%), but not in the lowest stretch duration (30-90 s). This indicates a dose-response relationship in the hip flexor muscles. Although the evidence is based on a small number of studies, this information will be of great importance for both athletes and coaches.
Keywords: flexibility; iliopsoas; mobility; rectus femoris.